Info

Bar magnet 1 Am2

Magnetization, M

Am"1

Sediments » 10'3 Am"1 ; Volcanics » 1 Am"1

Magnetic susceptibility, %

Dimensionless

Magnetite » 2.5

Permeability of free space, |x0

Hm"1

4n x 10"7 Hm"1

Fig. 2.1. The equivalence of a bar magnet and a current loop. When placed in a magnetic field B each suffers a torque according to its magnetic dipole moment m. The current loop has area A and current i flowing in the loop. The bar magnet has "pole strength" p and distance I between the imaginary poles at the ends of the magnet.

Fig. 2.1. The equivalence of a bar magnet and a current loop. When placed in a magnetic field B each suffers a torque according to its magnetic dipole moment m. The current loop has area A and current i flowing in the loop. The bar magnet has "pole strength" p and distance I between the imaginary poles at the ends of the magnet.

The magnetization of any material is generally made up of two components: the remanent magnetization (or simply remanence), which is that remaining in the absence of an applied field; and the induced magnetization, which is that induced by an applied field but which disappears after removal of that field. When dealing with rocks, the total magnetization M is made up of the vector sum of the remanence Mn and the magnetization Mj induced by the Earth's magnetic field, where

In isotropic materials the induced magnetization Mj lies along the direction of the applied field H (i.e., B) and is proportional to the magnitude of that field, that is

Where x is a constant of proportionality called the magnetic susceptibility. Since H and Mj have the same dimensions (2.1.1 and Table 2.1), x is a dimensionless number. Some banded sediments, layered intrusions, and foliated metamorphic rocks are magnetically anisotropic and have greater susceptibility in the plane of layering. These are special cases and most rocks used in paleomagnetism, such as basalts, dolerites, redbeds, and limestones, are magnetically isotropic or nearly so (see also §2.3.9).

The Koenigsberger ratio (0 has been defined as the ratio of the remanent to induced magnetization and is given by

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